Carbon fiber produced in a gaseous phase is known to have a high degree of preferred orientation of carbon layers, as well as excellent properties of high strength, high modulus and high corrosion resistance. Further, the carbon fiber has a graphite structure of substantially single crystal through heat treatment, thereby to form an ideal interlaminar compound, which is characterized by high conductivity comparable to copper or alminium and by good performance in, for example, occlusion of hydrogen and thus is potential as a functional material. As for mechanical properties, such carbon fiber has much higher performance than PAN (polyacrylonitrile)-based carbon fibers and pitch-based carbon fibers and thus can be an ideal structural material.
Heretofore, the carbon fiber according to a gaseous phase process has been produced by sequential steps of placing in an electric furnace a substrate of alumina or graphite, onto which an ultra-fine particle catalyst of iron, nickel or the like is formed, over which catalyst in turn is introduced a mixed gas consisting of a gas of hydrocarbon, such as benzene, and a carrier gas, such as hydrogen, thereby to decompose the hydrocarbon at the temperature of 1010.degree.-1300.degree. C. to form and grow the carbon fiber on the substrate. A general procedure for forming the ultra-fine particle catalyst on the substrate is to suspend ultra-fine metal particles of about 100 to 300 .ANG.in a volatile liquid, such as alcohol, having low surface tension, and to spray or apply the resulting suspension onto the substrate, which is then dried. Such procedure has several disadvantages as follows: (1) high irregularity of fiber length on account of temperature differences with resulting uneven fiber length and of ununiform spray of catalyst with resulting coarseness and closeness of the fiber formation, (2) as a gas of organic compound as a carbon source is consumed by reaction, there occurs a difference in a concentration of the organic compound between an inlet and an outlet to the reactor, so that sizes of the resulting fibers may vary in the vicinity of the inlet or the outlet, (3) as the carbon fiber is formed only on the surface of the substrate but the central zone of the reactor is not involved in the reaction, resulting in a poor yield, and (4) independent and separate processes, such as dispersion of the ultra-fine particles onto the substrate, placement of said substrate within a furnace, reduction of oxidized metal particles with hydrogen at the elevated temperature, formation of carbon fibers on the substrate through thermal decomposition of the organic compound, decrease of the furnace temperature, and removal of the carbon fibers, are required, so that continuous production is difficult and thus the productivity is low. Consequently, the production cost becomes too high to compete with already commercialized PAN-based carbon fibers and pitch-based carbon fibers except for special usage, for example, in the functional material.
It has now been found out that a mixed gas consisting of a gas of organic metal compound or compounds, a carrier gas and desirably a gas of carbon compound or compounds may be reacted at the elevated temperature to form the ultra-fine particle catalyst in a gaseous phase reaction and to produce the carbon fibers continuously and efficiently, instead of forming the ultra-fine particle catalyst of iron or nickel on the substrate as in the conventional process.
Accordingly, an object of the invention is to provide a continuous process for preparing carbon fibers in a gaseous phase reaction, which may solve the problems as described hereinabove and improve the productivity.